There are a number of procedures commonly used in the art for in vitro synthesis of nucleic acid molecules, including both DNA and RNA. For example, one may use an in vitro transcription reaction to synthesize RNA from a DNA template present in the reaction. T7-type RNA polymerases, such as T7 RNA polymerase, T3 RNA polymerase or SP6 RNA polymerase, are commonly used in such reactions, although many other RNA polymerases may also be used. Usually, but not always, synthesis of RNA is de novo (i.e., unprimed), and usually, but not always, transcription is initiated at a sequence in the template that is specifically recognized by the RNA polymerase, termed a "promoter" or a "promoter sequence". A method for in vitro transcription is presented herein.
Procedures for in vitro nucleic acid synthesis are also commonly used in the art to amplify nucleic acid molecules, including both DNA and RNA. For example, transcriptions using RNA polymerases are an integral part of "nucleic acid sequence-based amplification" (NASBA), "self-sustained sequence replication" (3SR), and "transcription-mediated amplification" (TMA) Hill, C. S., 1996, three similar methods for amplifying nucleic acid molecules in vitro.
By way of example, all or a specific portion of an RNA molecule may be amplified using NASBA (Compton, et al., 1991) or 3SR (Fahy, et al., 1991) by isothermal incubation of a sample RNA in a buffer containing two primers (a first primer complementary to the RNA molecule and encoding a promoter sequence for an RNA polymerase and a second primer complementary to the 3'-end of the first cDNA strand resulting from reverse transcription of the RNA molecule), an RNA- and DNA-dependent DNA polymerase which also has RNase H activity (or a separate RNase H enzyme), all four canonical 2'-deoxynucleoside-5'-triphosphates (dATP, dCTP, dGTP and dTTP), an RNA polymerase that recognizes the promoter sequence of the first primer, and all four canonical ribonucleoside-5'-triphosphates (rATP, rCTP, rGTP and rUTP).
A first cDNA strand is synthesized by extension of the first primer by reverse transcription. Then, the RNase H digests the RNA of the resulting DNA:RNA hybrid, and the second primer primes synthesis of the second cDNA strand. The RNA polymerase then transcribes the resultant double-stranded DNA (ds-DNA) molecule from the RNA polymerase promoter sequence, making many more copies of RNA, which in turn, are reversed transcribed into cDNA and the process begins all over again. This series of reactions, from ds-DNA through RNA intermediates to more ds-DNA, continues in a self-sustained way until reaction components are exhausted or the enzymes are inactivated. DNA samples can also be amplified by other variations of NASBA or 3SR or TMA.
Another nucleic acid amplification method involving DNA synthesis is the polymerase chain reaction (PCR).
By way of example, a specific portion of a DNA molecule may be amplified using PCR by temperature cycling of a sample DNA in a buffer containing two primers (one primer complementary to each of the DNA strands and which, together, flank the DNA sequence of interest), a thermostable DNA polymerase, and all four canonical 2'-deoxynucleoside-5'-triphosphates (dATP, dCTP, dGTP and dTTP). The specific nucleic acid sequence is geometrically amplified during each of about 30 cycles of denaturation (e.g., at 95.degree. C.), annealing of the two primers (e.g., at 55.degree. C.), and extension of the primers by the DNA polymerase (e.g., at 70.degree. C.), so that up to about a billion copies of the nucleic acid sequence are obtained. RNA may be similarly amplified using one of several protocols for (reverse transcription-PCR) RT-PCR, such as, for example, by carrying out the reaction using a thermostable DNA polymerase which also has reverse transcriptase activity (Myers and Gelfand, 1991).
The polymerase chain reaction, discussed above, is the subject of numerous publications and patents, including, for example: Mullis, K. B., et al., U.S. Pat. No. 4,683,202 and U.S. Pat. No. 4,965,188.
A variety of procedures for using in vitro nucleic acid synthesis for characterizing nucleic acid molecules, including both DNA and RNA, also are known in the art.
There are many reasons for characterizing nucleic acid molecules. For example, genes are rapidly being identified and characterized which are causative or related to many human, animal and plant diseases. Even within any particular gene, numerous mutations are being identified that are responsible for particular pathological conditions. Thus, although many methods for detection of both known and unknown mutations have been developed (e.g., see Cotton, 1993), our growing knowledge of human and other genomes makes it increasingly important to develop new, better, and faster methods for characterizing nucleic acids. Besides diagnostic uses, improved methods for rapidly characterizing nucleic acids will also be useful in many other areas, including human forensics, paternity testing, animal and plant breeding, tissue typing, screening for smuggling of endangered species, and biological research.
One of the most informative ways to characterize a DNA molecule is to determine its nucleotide sequence. The most commonly used method for sequencing DNA at this time (Sanger, et al., 1977) uses a DNA polymerase to produce differently sized fragments depending on the positions (sequence) of the four bases (A=Adenine; C=Cytidine; G=Guanine; and T=Thymine) within the DNA to be sequenced. In this method, the DNA to be sequenced is used as a template for in vitro DNA synthesis. RNA may also be used as a template if a polymerase with RNA-directed DNA polymerase (i.e., reverse transcriptase) activity is used. In addition to all four of the deoxynucleotides (dATP, dCTP, dGTP and dTTP), a 2',3'-dideoxynucleotide is also included in each in vitro DNA synthesis reaction at a concentration that will result in random substitution of a small percentage of a normal nucleotide by the corresponding dideoxynucleotide. Thus, each DNA synthesis reaction yields a mixture of DNA fragments of different lengths corresponding to chain termination wherever the dideoxynucleotide was incorporated in place of the normal deoxynucleotide.
The DNA fragments are labelled, either radioactively or non-radioactively, by one of several methods known in the art and the label(s) may be incorporated into the DNA by extension of a labeled primer, or by incorporation of a labelled deoxy- or dideoxy- nucleotide. By carrying out DNA synthesis reactions for each of the four dideoxynucleotides (ddATP, ddCTP, ddGTP or ddTTP), then separating the products of each reaction in adjacent lanes of a denaturing polyacrylamide gel or in the same lane of a gel if different distinguishable labels are used for each reaction, and detecting those products by one of several methods, the sequence of the DNA template can be read directly. Radioactively-labelled products may be read by analyzing an exposed X-ray film. Alternatively, other methods commonly known in the art for detecting DNA molecules labelled with fluorescent, chemiluminescent or other non-radioactive moieties may be used.
Because 2',3'-dideoxynucleotides (ddNTPs), including even ddNTPs with modified nucleic acid bases, can be used as substrates for many DNA polymerases, Sanger's dideoxy-sequencing method is widely used. Recently, Tabor and Richardson (EP application 942034331, 1994) reported that mutations at specific sites in many DNA polymerases improved the ability of these mutant enzymes to accept ddNTPs as substrates, thereby leading to improved DNA polymerases for DNA sequencing using the Sanger method.
Nucleic acid sequencing provides the highest degree of certainty as to the identity of a particular nucleic acid. Also, nucleic acid sequencing permits one to detect mutations in a gene even if the site of the mutation is unknown. Sequencing data may even provide enough information to permit an estimation of the clinical significance of a particular mutation or of a variation in the sequence.
Cycle sequencing is a variation of Sanger sequencing that achieves a linear amplification of the sequencing signal by using a thermostable DNA polymerase and repeating chain terminating DNA synthesis during each of multiple rounds of denaturation of a template DNA (e.g., at 95.degree. C.), annealing of a single primer oligonucleotide (e.g., at 55.degree. C.), and extension of the primer (e.g., at 70.degree. C.).
Other methods for sequencing nucleic acids are also known besides the Sanger method. For example, Barnes described a method for sequencing DNA by partial ribo-substitution (Barnes, W. M., 1977). In this method, a pre-labelled primer was extended in vitro on a template DNA to be sequenced in each of four reactions containing a wild-type DNA polymerase in the presence of Mn2+, all four canonical 2'-deoxyribonucleoside triphosphates, and one of four ribonucleoside triphosphates under deoxy-/ribonucleotide ratios and conditions that result in about 2% ribonucleotide substitution at each position. After alkali treatment to cleave the synthetic DNA at the positions of partial ribosubstitution, the sequence was determined by analyzing the fragments resulting from each reaction following electrophoresis on a denaturing polyacrylamide gel.
Although most methods for sequencing nucleic acids employ DNA polymerases, some work has also been reported on the use of T7 RNAP and SP6 RNAP for transcription sequencing of DNA templates beginning at the respective T7 or SP6 promoter sequence using 3'-deoxyribonucleoside-5'-triphosphates (Axelrod, V. D., and Kramer, F. R., 1985), and Q-Beta replicase for sequencing single-stranded RNA templates (Kramer, F. R., and Mills, D. R., 1978). Also, 3'-O-methyl-ribonucleoside-5'-triphosphates have been used for sequencing DNA templates with E. coli RNA polymerase ((Axelrod, V. D., et al., 1978). None of these techniques is commonly used at present, perhaps in part, due to the difficulty to obtain the 3'-deoxy- and 3'-O-methyl-nucleoside triphosphate substrates, while 2',3'-dideoxyribonucleoside-5'-triphosphates that are commercially available have not been found to be substrates for wild-type (w.t.) RNA polymerases.
In view of the numerous applications involving in vitro nucleic acid synthesis known in the art, it is useful to consider the properties of the key nucleic acid polymerase reagents which make these procedures possible, and which, if modified in their essential properties, might improve these procedures.
One classification of nucleic acid polymerases relies on their different template specificities (RNA or DNA), substrate specificities (rNTPs or dNTPs), and mode of initiation (de novo or primed). These designations usually refer to the template and substrate specificities displayed in vivo during the fulfillment of a polymerase's biological function.
In vitro, polymerases can display novel activities, albeit with reduced efficiency and/or under non-physiological conditions. E. coli DNA-directed DNA polymerase I, for example, can use RNA as a template, although there is a concomitant .sup..about. 100-fold increase in dNTP K.sub.m (Ricchetti and Buc, 1993). T7 DNA-directed RNA polymerase can also use RNA as a template (Konarska and Sharp, 1989). These are not exceptional observations because it is a general property of polymerases that they display relaxed template specificity, at least in vitro.
While template specificity may be relaxed, polymerase substrate specificity is normally extremely stringent. T7 DNAP, for example, displays at least 2,000-fold selectivity for dNTPs over rNTPs, even in Mn.sup.++ buffer which relaxes the ability of the polymerase to discriminate between dNTPs and ddNTPs (Tabor and Richardson, 1989).
It has been reported that transcripts synthesized by a T7 RNAP Y639F mutant in vivo yielded 1/2-1/3 of the protein per transcript compared to transcripts synthesized by the wild-type enzyme (Makarova, et al., 1995). The latter phenotype was unique to the Y639F mutant amongst a number of other active site mutants examined for in vivo expression, and indicated that Y639F transcripts contained a defect that led to their being inefficiently translated.
A polymerase with an altered substrate specificity would be useful in many molecular biological applications, such as creating a nucleic acid molecule comprising at least one non-canonical nucleotide.